1. Field of the Invention
This invention relates to a transmission apparatus which can be used for satellite broadcasting and communication in sending and receiving multiplex signals.
2. Description of the Prior Art
A method of multiplex transmission has been proposed to send sub video signals, such as additional information for higher definition and an increased aspect ratio, without interfering with conventional satellite broadcasting receivers. This transmission method modulates the frequency of a main carrier using a baseband signal obtained by modulating by means of a sub video signal the picture subcarrier with a frequency set higher than those of the main video signal and audio subcarrier. However, this method, which has already been described in U.S. patent application Ser. No. 07/268,966 field on Nov. 9, 1988, now U.S. Pat. No. 5,061,999, is accompanied by a problem of an occurrence of crosstalk from the main video signal to the sub video signal.
The manner in which such crosstalk occurs is described below.
The main carrier transmission line amplitude and group-delay frequency responses are represented by the sum of nth-order polynomials with the frequencies as its variable whose origin is the central frequency of the transmission line. In such cases, the coefficients of the amplitude response's 3rd-order and group-delay response's 1st-order terms are associated with the 2nd-order intermodulation generated in the baseband signal (modulation signal) transmitted. This mechanism has been disclosed in FM Musen Kogaku pp 546-552, Sugawara et al., Nikkan Kogyo Shinbunsha.
This intermodulation becomes particularly higher when the bandwidth of a transmission line is smaller than the occupied bandwidth of a main carrier specified by the Carson's rule. Here, the BW, or the occupied bandwidth of a frequency modulation signal specified by the Carson's rule, is given by the following equation: EQU BW=f+2 fm (1)
where .DELTA.f represents the maximum frequency deviation of the main carrier and fm represents the highest frequency of the modulation signal. At present, satellite broadcasting uses the transmission line bandwidth of 27 MHz and .DELTA.f of 17 MHz p-p, with fm of 10 MHz for the multiplex signal. This calculates the BW to be 37 MHz, raising a problem of the 2nd-order intermodulation taking place in a modulation signal.
Meanwhile, the main carrier transmission line amplitude and group-delay responses depend largely on the band-pass filter (BPF) characteristics of a transmitter, a receiver, and a satellite transponder. If the main carrier is in the center of the BPF and the amplitude and group-delay responses of the BPF are axis-symmetric, no amplitude response's 3rd-order and group-delay response's 1st-order terms develop. This means there is no occurrence of 2nd-order intermodulation in the modulation signal. However, if the main carrier deviates from the center due to frequency modulation, the amplitude response's 3rd-order and groupdelay response's 1st-order terms develop, thereby generating the 2nd-order intermodulation in the modulation signal.
In the frequency modulation, the instantaneous voltage of a modulation signal determines the instantaneous frequency of the carrier. Here, the modulation signal is a multiplex signal where an audio and a picture subcarrier are superimposed on the main video signal and, therefore, the main video signal determines the bias point of the main carrier on the transmission line. The averaging automatic frequency control (AFC) method used by the existing satellite broadcasting for the transmission of a main carrier sets the frequency f0 corresponding to the DC level v0 (averaging picture level, APL) of a modulation signal to be equal to the central frequency of the BPF. In consequence, the magnitude of the 2nd-order intermodulation generated depends upon the difference between f0 and fi, an instantaneous frequency determined by the instantaneous voltage vi of the main video signal. If the keyed AFC method is used for the transmission of a main carrier, the magnitude of the 2nd-order intermodulation becomes a function of the difference between fi and the frequency f0' corresponding to the voltage v0' in the keyed section of the main video signal when f0' is so set as to be in the center of the BPF.
If the picture subcarrier frequency is taken as fs and an arbitrary component of the main video signal frequency spectrum is taken as fb, the position of the frequency in the modulation signal at which the 2nd-order intermodulation occurs is represented by fs.+-.fb. In this configuration, therefore, the main video signal spectrum is arranged on both sides of the picture subcarrier positioned in the center. The 2nd-order intermodulation amplitudes Vfs+fb and Vfs-fb in the frequency fs.+-.fb are given by the following equations as functions of fi-f0 or vi-v0: ##EQU1## where UFi represents, by way of a complex number, the amplitude and phase responses of the transmission line in the frequency Fi with the center of the transmission line as the origin. mfs and mfb are modulation indices of the main carrier in the frequencies fs and fb, and .DELTA.fm is the maximum frequency deviation of the main carrier. Jn(x) is a nth-order Bessel function. This theory has been described in FM Musen Kogaku pp 538-539, Sugawara et al., Nikkan Kogyo Shinbunsha.
This 2nd-order intermodulation in the modulation signal interferes with a picture subcarrier.
First, in the case of frequency modulation of a picture subcarrier, the amplitude Vc of an interference occurring to the sub video signal may be represented by the following equation when the ratio of the amplitude U of 2nd-order intermodulation to the amplitude D of the subcarrier is small: EQU Vc=(1/.DELTA.fs)(U/D)(fu-fd)cos2.pi.(fu-fd)t (4)
where fd is the instantaneous frequency of a picture subcarrier, fu is the instantaneous frequency of a 2nd-order intermodulation, and .DELTA.fs is the maximum frequency deviation of a subcarrier. This theory has been disclosed in FM Musen Kogaku pp 456-457 and p 604, Sugawara et al., Nikkan Kogyo Shinbunsha. Substitution of the Vfs-fd and Vfs-f derived from the equations (2) and (3) for the equation (4) gives the following equation: EQU Vc=KFM.multidot.fb.multidot.cos2.pi. fb t (5),
where KFM equals (1/.DELTA.fs)(Vfs+fb-Vfs-fb)/D. According to the equation (5), a main video signal with a frequency of fb also generates on the sub video signal an interference with a frequency of fb, whose magnitude is proportional to the amplitude of the 2nd-order intermodulation. Any fluctuations in the picture subcarrier frequency due to frequency modulation cause no change in the interference signal as the difference in frequency between the 2nd-order intermodulation and the picture subcarrier remains unchanged. The interference due to each frequency component of the main video signal is added on the sub video signal to cause crosstalk from the main to the sub video signal. However, each component of the interference calculated by multiplying the frequency in the equation (5) brings a differential waveform of the main signal to the crosstalk. The crosstalk is subject to an attenuation due to the coefficient term KFM. The value of this coefficient term is determined by the bias point of the main carrier on the transmission line. This bias point is determined by the main video signal as mentioned earlier.
Next, a case is considered where a picture subcarrier undergoes phase modulation. A 2nd-order intermodulation in the modulation signal causes the phase .theta. of the picture subcarrier to fluctuate. The amplitude of Vc of the interference occurring to the sub video signal may be represented by the following equation when the ratio of the amplitude U of the 2nd-order intermodulation to the amplitude D of the picture subcarrier is small: EQU Vc=(1/.DELTA..theta.)(U/D) cos2.pi.(fu-fd)t (6),
where .DELTA..theta. is the maximum phase deviation of the picture subcarrier. This theory has been described in FM Musen Kogaku pp 449-451, Sugawara et al., Nikkan Kogyo Shinbunsha. Substitution of Vfs+fb and Vfs-fb derived from the equations (2) and (3) for the equation (6) gives the following equation: EQU Vc=KPM.multidot.cos2.pi. fb t (7),
where KPM equals (1/.DELTA..theta.)(Vfs+fb-Vfs-fb)/D. According to the equation (7), a main video signal with a frequency of fb also generates on the sub video signal an interference with a frequency of fb, whose magnitude is proportional to the amplitude of the 2nd-order intermodulation. The interference due to each frequency component of the main video signal is added on the sub video signal to cause crosstalk from the main video signal to the sub video signal. The crosstalk is also subject to an attenuation due to the coefficient term KPM. The value of this coefficient term is determined by the bias point of the main carrier on the transmission line. This bias point is determined by the main video signal as mentioned earlier.
Then, a case is considered of amplitude modulation of a picture subcarrier. A 2nd-order intermodulation in the modulation signal causes fluctuations of the subcarrier's amplitude D, which may be represented by the following equation when the ratio of the amplitude U of the 2nd-order intermodulation to the amplitude D is small: EQU D'=D{1+(U/D) cos2.pi.(fu-fd)t} (8).
This theory has been disclosed in FM Musen Kogaku pp 447-449, Sugawara et al., Nikkan Kogyo Shinbunsha. According to the equation (8), the picture subcarrier is subjected to amplitude modulation by the frequency .vertline.fu-fd.vertline. and the modulation degree U/D. The crosstalk signal Vc may, therefore, be represented by the following equation using the equation (8) and Vfs+fb and Vfs-fb derived from the equations (2) and (3): EQU Vc=KAM.multidot.cos2.pi. fb t (9),
where KAM equals (Vfs+fb-Vfs-fb)/D. According to the equation (9), the main video signal with a frequency of fb also generates on the sub video signal an interference with a frequency of fb, whose magnitude is proportional to the amplitude of the 2nd-order intermodulation. The interference due to each frequency component of the main video signal is added on the sub video signal to cause crosstalk from the main to the sub video signal. The crosstalk is also subject to an attenuation due to the coefficient term KAM. The value of this coefficient term is determined by the bias point of the main carrier on the transmission line. This bias point is determined by the main video signal as mentioned earlier.
A similar phenomenon that has so far been known is the crosstalk from the main video signal to the sub audio signal that takes place at the time of multiplex propagation of an FM--FM multiplexed TV sound multiplex broadcasting wave. In order to improve this crosstalk, a method has been proposed where a crosstalk correction signal prepared by letting an FM--FM multiplex wave pass through an AM demodulator, a differentiator, an attenuator, and an inverter is added to the sub audio signal on the strength of an assumed advantage that the differential waveform of an FM--FM multiplex wave envelope curve is similar to crosstalk. Such a method has been disclosed in the Japanese Patent Publication No. 57-26469 (1982).
In satellite transmission, however, it is difficult to obtain a correction signal from an FM multiplex wave envelope curve. There are two reasons for this. The first reason is that the nonlinear amplification behavior of a transponder TWTA within a satellite distorts the information on the amplitude of an FM multiplex wave. The second is that, in satellite transmission, the very small power to transmit an FM multiplex wave lowers the C/N ratio at the time a signal is received, allowing the AM demodulated output of the FM multiplex wave to give a much lower S/N ratio than the demodulated video signal transmitted by FM. Hence, such a conventional method of reducing crosstalk has not applicable in the case of satellite transmission.